It has been postulated that in vivo toxicity of asbestos and air pollution particles results from the catalysis of free radical generation. Exposure to these lung toxins can be associated with increased human morbidity and mortality. However, molecular mehcanisms of lung injury remain unknown. We examined in vivo free radical generation after exposure to asbestos or air pollution particles by using electron spin resonance (ESR) spin trapping technique. ESR spectroscopy of the chloroform extract from lungs exposed to asbestos or air pollution particles gave a spectrum consistent with a carbon-centered radical adduct, while those spectra from control lungs revealed a much weaker signal. The radical adducts detected by ESR are very similar to, if not identical, with ethyl and penthyl radical adducts, providing for the first time ESR evidence of in vivo lipid peroxidation resulting from asbestos and air pollution particle exposure. In addition, the generation of free radicals in lung tissue appears to be associated with soluble metals present in the air pollution particles. Understanding the source of radical species could lead to an understanding of the mechanisms underlying clinical pathology in patients.